Is there a reason why the ImportManager and ExportManager only handles a maximum of 3 product images when importing or exporting? As I understand, there is no restriction as to how many images a product can have assigned to it, so I don't really understand why we impose a restriction in the import and export.
If there is no specific reason then I would suggest that we change the source code to allow any number of picture properties to be imported/exported from/to xlsx and xml. I would also suggest that we set a minimum number of picture property fields (say 3) to be exported so when there are no pictures to export, the empty fields are still exported as placeholders.